In many optical articles including various displays such as computer displays, TV and plasma displays, liquid crystal display devices, transparent plastic lenses, covers in various instruments, and window panes in automobiles and trains, antireflective films are often used as the outermost layer for the purpose of improving visibility. The antireflection principle requires that the antireflective film have a low refractive index.
Fluoroplastics are employed in the antireflective application as in displays because they essentially have a low refractive index and satisfactory alkali resistance. However, the fluoroplastics are often used as rubber due to their molecular structure and unsuitable to form hard protective coatings having mar resistance.
Recently, hydrolyzable silane compounds having perfluoroalkyl groups were developed. To take advantage of their favorable characteristics, a variety of coating compositions comprising hydrolyzable silane compounds have been developed for imparting alkali resistance, water repellency, oil repellency, anti-staining and anti-reflection. However, since perfluoroalkyl groups contributing to these favorable characteristics are bulky and inert, the cured coatings have a low crosslink density. As a result, the cured coatings are fairly hard as compared with fluoroplastics, but still insufficient in mar resistance.
For the purpose of enhancing mar resistance, JP-A 2000-119634 proposes co-hydrolysis of a perfluoroalkyl-containing silane and a silane compound such as a tetraalkoxysilane; and JP-A 2004-315712 proposes a material based on a bissilane compound having perfluoroalkylene as a spacer. These systems achieve satisfactory levels of mar resistance and adhesion, but are less antireflective because of a less reduction of refractive index.
In view of the fact that the material having the lowest refractive index is air, hollow inorganic fine particles are devised for the purpose of taking air into the structure of a cured coating. One proposal is a porous or hollow silica sol (JP-A H07-133105 and JP-A 2001-233611). On use, the silica sol is mixed with a fluorinated alkyl-containing silicone (JP-A 2002-79616) or dispersed in a binder component containing an ionizing radiation curable monomer (JP-A 2004-272197). In the follow-up test, the inventors found that when the hollow silica sol is mixed with an organic solvent solution of any of the foregoing binders, uniform dispersion is observed, but upon volatilization of the organic solvent, the hollow silica particles emerge and float on the coating surface due to their internal voids. As a result, the cured coating as a whole displays satisfactory antireflection properties, but unsatisfactory mar resistance because many hollow silica particles which are weak in strength on account of the hollow structure are present on the surface and loosely bound by the binder component. In addition, the cured coating has poor alkali resistance as the drawback of silica.
For the purpose of overcoming the drawback of hollow silica sol, an attempt is made to surface treat hollow silica particles with a fluorinated alkyl-containing silane compound and mix and disperse the hollow silica particles in a binder derived from tetraalkoxysilane (JP-A 2005-266051). Although the coverage of the particle surface with hydrophobic groups improves alkali resistance, this method relying only on mixing step is not successful in preventing hollow silica particles from emerging, failing to improve mar resistance. In another attempt, a hollow silica sol is surface treated with a silane compound containing an ionizing radiation-polymerizable group before it is dispersed in an ionizing radiation curable resin (JP-A 2005-99778). Although silica particles are bound by the binder when cured, this method yet fails to prevent silica particles from emerging prior to curing, resulting in poor mar resistance.
For the purpose of improving mar resistance, JP-A 2007-146106 proposes to integrate hollow inorganic oxide particles with a hydrolyzate of a bissilane compound of specific structure to form a composite resin. Then hollow inorganic oxide particles can be uniformly dispersed in the cured coating without allowing hollow inorganic oxide particles to emerge afloat. The composite resin meets both mechanical strength (mar resistance) and a low refractive index (antireflection). However, curing requires a temperature above 100° C., which can cause deformation of the substrate. There is a desire for a system that does not require a temperature above 100° C. for curing.
This may be achieved by designing a photocurable system. However, acrylic or epoxy groups are necessary for photo-crosslinking. These functional groups serve to increase a refractive index. Even when hollow inorganic oxide particles are incorporated, the refractive index cannot be reduced and the reflectance cannot be reduced below a certain limit.